The present invention relates to a breathing apparatus or ventilator system for supplying gases to a living being such as a human or animal patient to assist the patient in breathing, and is particularly concerned with a control system and method for controlling switching of the ventilator from gas delivery (inhalation) to gas removal (expiration).
There are two types of ventilation used to assist breathing in certain patients. One is complete control of breathing, in which the ventilator completely takes over the patient""s breathing function. The other partially supports the patient""s breathing function, such as pressure support ventilation, or PSV, in which the ventilator simply assists the patient""s own breathing function. PSV is generally more comfortable for the patient, as long as the ventilator can be synchronized with the patient""s natural breathing effort. However, accurate synchronism of the ventilator with the patient is a long-standing problem in PSV.
In known pressure support ventilators, a breathing apparatus is connected to a patient through an inspiration line, for supplying gas to the patient, and an expiratory line, for allowing removal of expired gases from the patient. A valve in each line controls the inspiration phase and the expiration phase of the breathing cycle, and the opening and closing of each valve are controlled by a control unit based on a desired pressure target level. Typically, during pressure support ventilation, the breathing apparatus starts to deliver gases to the patient when a spontaneous breathing effort is detected by a pressure gauge or flow sensor. The ventilator flow in inspiration is normally terminated when the inspiratory flow rate decays to a predetermined percentage of the peak inspiratory flow rate, typically between 5% to 25% of the peak inspiratory flow rate. This percentage is commonly known as the termination criterion or the expiratory trigger sensitivity, or ETS. In some cases, the termination of inspiration can be selected manually by the user. In each of these cases, the termination criterion, or ETS, is always a fixed number or percentage, regardless of variations between patients or changes in the condition of an individual patient over time.
Clinical studies have revealed that patients under pressure support ventilation, or PSV, often encounter patient-ventilator asynchrony. In expiratory asynchrony, the termination of ventilator flow occurs either prematurely, before the patient stops his inspiratory effort, or late, after the patient stops his inspiratory effort. In either case, discomfort to the patient is caused. When the termination is late (delayed termination), the patient recruits his expiratory muscles to xe2x80x9cfightxe2x80x9d against the ventilator flow, increasing expiratory workload. If the termination is too early (premature termination), inspiratory muscle work continues into or even through the ventilator""s expiratory phase, resulting in inefficient respiratory muscle work. Although expiratory asynchrony has been of clinical concern for years, no effective solution to this problem has yet been devised. This problem is discussed in the paper entitled xe2x80x9cVariability of Patient-Ventilator Interaction with Pressure Support Ventilation in Patients with Chronic Obstructive Pulmonary Diseasexe2x80x9d by Jubran, A., et al., Am.J.Respir Crit Care Med 152:129-136, 1995. In spite of the known problem of ventilator expiratory asynchrony, most current ventilators still use an arbitrary termination criterion, such as a fixed percentage of the peak flow, in order to terminate the inspiratory flow delivery during PSV. Thus, the Siemens Servo 300 uses 5% of the peak flow as the termination criterion, Siemens Servo 900 and Bird 8400St use 25% of the peak flow, and the Nellcor Puritan Bennett 7200ae uses a flow rate of 5 L/min.
One attempt to avoid the problems of patient-ventilator asynchrony has been to allow the expiratory trigger sensitivity to be adjusted by the doctor or medical personnel. However, this adds significantly to the workload of medical personnel and it has been shown to be difficult to achieve good results based on patient observation at the bedside.
It is an object of the present invention to provide a new and improved pressure support ventilation control system and method which reduces the risk of patient-ventilator asynchrony.
According to one aspect of the present invention, a pressure support ventilation system is provided, which comprises a source of breathing gases, an inspiration line for connecting the source to a patient, an expiratory line for exhausting gases from the patient, at least one flow sensor for sensing the level of gas flow in the system, a pressure sensor for sensing pressure in the system, and a control unit for controlling supply of gases to a patient in an inspiration cycle, and exhausting gases in the expiratory line in an exhalation cycle, the control unit being connected to the flow sensor and pressure sensor, and calculating, for each breath, a patient respiratory time constant and varying an expiratory trigger sensitivity at which the ventilator switches from inspiration to expiration in response to variations in the patient respiratory time constant, such that the expiratory trigger sensitivity is increased with increases in the respiratory time constant.
Preferably, the control unit or microprocessor is also programmed to calculate the supra-plateau pressure at the end of inhalation, i.e. the pressure above a set target pressure level at the end of inhalation or inspiration, and to adjust the expiratory trigger sensitivity if the supra-plateau pressure is too high.
This system provides automatic closed loop control of the expiratory trigger sensitivity (ETS), i.e. the ratio of inspiratory flow rate to the peak inspiratory flow rate at which the ventilator will switch from a gas delivery (inspiration) phase to a gas removal (expiration) phase. The system uses patient respiratory time constant, which has been found to have an excellent correlation with ventilator synchrony, in order to automatically adjust the expiratory trigger ratio or sensitivity for each breath in a closed-loop manner.
According to another aspect of the present invention, a method of controlling the inhalation phase in a pressure support ventilation system is provided, which comprises the steps of supplying gas to a patient in a ventilator inspiration phase, controlling the termination of the inspiration phase based on a calculated expiratory trigger sensitivity, detecting the flow rate and pressure in the system for each breath, using the detected flow rate and pressure to calculate, for each breath, a new patient respiratory time constant, and determining, for each calculated time constant, a new expiratory trigger ratio, and using the new expiratory trigger sensitivity to control the termination of the inspiration phase in the next breath.
Preferably, a supra-plateau pressure is also calculated for each breath, and this is also used to vary the expiratory trigger sensitivity based on predetermined criteria. Essentially, the higher the patient respiratory time constant, the higher the expiratory trigger ratio should be in order to achieve a good expiratory synchrony between the patient and ventilator during pressure support ventilation. Also, the relationship between the supra-plateau pressure and the expiratory trigger sensitivity is such that, the higher the supra-plateau pressure, the higher the expiratory trigger sensitivity should be for the next breath. The expiratory trigger sensitivity may be varied based on look-up tables which tabulate the desired relationship between patient respiratory time constant and expiratory trigger sensitivity, and between supra-plateau pressure and expiratory trigger sensitivity.
This closed loop control system and method for automatically varying the expiratory trigger sensitivity (ratio of inspiratory flow rate to the peak inspiratory flow rate at which the ventilator will switch from gas delivery or inspiration to gas removal or expiration) will achieve better patient-ventilator expiratory synchrony than was possible in previous systems with fixed expiratory trigger levels or ratios. This will result in reduced discomfort in patients undergoing pressure support ventilation or PSV.